This invention relates to an X-ray image intensifier for converting an X-ray image into a visible image.
X-ray image intensifiers are being used widely in X-ray image pickup apparatus for medical use and industrial televisions for non-destructive inspection.
This type of X-ray image intensifier has a vacuum envelope. This vacuum envelope is provided with an input window, through which X-rays are incident on the vacuum envelope. In the vacuum envelope, a curved substrate is placed facing the input window. An input fluorescent screen and a photoelectric layer are deposited in that order on the side of the substrate opposite to the input window. An anode and an output fluorescent screen are provided on the output side of the vacuum envelope. A focusing electrode is provided on the internal peripheral wall of the vacuum envelope.
The X-rays emitted from an X-ray tube penetrate the test object, pass through the input window and the substrate and are converted into light rays by the input fluorescent screen. The light rays are converted by the photoelectric layer into electrons. The electrons are accelerated and focused by an electron lens formed by the focusing electrode and the anode. Then, the electrons are converted by the output fluorescent screen into a visible image.
The visible image is picked up by using a TV camera, or a cinecamera or a spot camera to produce a permanent image, and the resultant image is then used for medical diagnosis, for example.
Among the input fluorescent screens used for X-ray image intensifiers lately is an input fluorescent screen which is far greater in thickness than the prior input fluorescent screens.
The X-rays absorbed by an input fluorescent screen with thickness T can be expressed as EQU l-e.sup..phi.T
where .phi. is the X-ray absorption coefficient. FIG. 1 shows the relation between the thickness of the input fluorescent screen and the X-ray absorption rate. In the figure, the material of the input fluorescent screen is cesium iodide (CsI) and an energy of X-rays is 60 keV. The X-ray absorption rate increases as the thickness increases. By increasing the X-ray absorption rate in this way, the X-rays can be utilized more effectively, making it possible to reduce the radiation dose and improve the quality of an image.
If uniform X-rays are irradiated to an X-ray image intensifier and an output image is observed, it sometimes causes the central portion of the output image to be light and the brightness to be decreased toward the peripheral areas. The reason is that the peripheral areas of the image is enlarged more than the central part by what is called an electron lens of the X-ray image intensifier. With such an output brightness distribution, it is impossible to make an effective use of the whole dynamic range after an image is picked up. That is to say, a wide usable range of an output image cannot be secured.
As one of the methods for making the output brightness distribution as flat as possible, there is a known method that increases the thickness of the input fluorescent screen from the central part progressively toward the peripheral areas, as disclosed in Japanese Patent Disclosure No. 78-102663. With this method, the input fluorescent screen absorbs more X-rays and emits more light at the peripheral areas than the central part. Therefore, the brightness of the peripheral areas is increased on the output side and the output brightness distribution can thereby be made close to a flat distribution.
This means cannot be applied to an X-ray image intensifier incorporating a thickness-increased input fluorescent screen described above. The reason is described in the following. First, let us consider using a model how much of the light emanating from the input fluorescent screen reaches the photoelectric layer when a certain quantity of X-rays are falls on the input fluorescent screen. The model is shown in FIG. 3. In an input fluorescent screen with thickness T, the quantity of conversion of X-rays into light at a micro part dt at the depth t is proportional to the dose of X-rays at the position t. Since the distance from the micro part dt to the photoelectric layer is T-t, if the attenuation coefficient of the light in the input fluorescent screen is denoted by .beta., the quantity of light that reaches the photoelectric layer of all the light produced by conversion at the micro part dt is: EQU .alpha.e.sup.-.alpha.T. e.sup.-.beta.(T-t) dt
Therefore, by integrating the above equation, the quantity of light reaching the photoelectric layer of all the light to which the X-rays are converted over the whole input fluorescent Screen is given as follows. ##EQU1## where .alpha. denotes the X-ray absorption coefficient. This definite integral has a peak value. Input fluorescent screens of various thicknesses were produced and the quantity of light of the photoelectric layers was measured. The light quantity of the photoelectric layer showed a peak (maximal) value at a certain thickness. The experimental results are shown in Fig, 4. The data used for the curve were measured values of the brightness of independent input fluorescent screen films composed of CsI. The energy of the X-rays in this experiment was 60 keV.
If, in order to make good use of the X-rays, a thickness value at which a peak value of light quantity is obtained is used for the thickness of the central part of an input fluorescent screen, the earlier-described method of correcting the output brightness distribution cannot be applied. To be more specific, evenif the peripheral areas of the input fluorescent screen is increased in thickness than the central part, the brightness of the peripheral areas is lower. As a result, the graph of output brightness distribution assumes a sharp-peaked normal distribution curve. If the thickness is increased further, the resolution is reduced due to the dispersion of the light. Therefore, a thickness corresponding to a peak value of the quantity of light produced is considered as the maximum thickness that can be applied for practical use. Hence, when such a thick film type input fluorescent screen is made, there arises a problem that the output brightness distribution cannot be corrected effectively and this problem must be solved.
Another problem will be described in the following. If the thickness is varied over the whole area of the screen, the X-ray absorption coefficient changes with the quality of X-ray at different positions of the screen. For this reason, even if the output brightness distribution is flat with a given quality of X-ray, the distribution is not flat with another quality of X-ray.
As the other way of making the output brightness distribution flat, there is a method of forming a film, the light transmittance of which is varied, over the whole area of the film on the surface of the input fluorescent screen. More specifically, this method uses a reduced light transmittance for the part of the film at the center of the input fluorescent screen thereby flattening the output brightness distribution. However, this method s accompanied by a problem that some processes have to be added for vapor-depositing a film having a light transmittance varied in a symmetric form. Since there is a symmetric variation in the light transmittance of the film between the input fluorescent screen and the photoelectric layer, the conditions for forming the photoelectric layer are not uniform. In addition, there is a possibility that a symmetric variation occurs in the variation with time.